The latest developments of the 3GPP standards are referred to as the Long Term Evolution (LTE) of EPC (Evolved Packet Core) network and E-UTRAN (Evolved UMTS Terrestrial Radio Access Network). Under the 3GPP standards, a NodeB (or an eNB in LTE) is the base station via which communications devices connect to a core network and communicate to other communications devices or remote servers. For simplicity, the present application will use the term base station to refer to any such base stations. In the core network, a home subscriber server (HSS) entity holds information related to the services subscribed/available for each communication device. Mobility of the communication devices (i.e. access to the core network via various radio access networks) is managed by a so-called mobility management entity (MME). The MME communicates with the communication devices using non-access stratum (NAS) protocol messages.
Communications devices might be, for example, mobile communications devices such as mobile telephones, smartphones, user equipment, personal digital assistants, laptop/tablet computers, web browsers, e-book readers and the like. Such mobile (or even generally stationary) devices are typically operated by a user. However, 3GPP standards also make it possible to connect Machine-Type Communications (MTC) devices (sometimes also referred to as Machine-to-Machine (M2M) communications devices) to the network, which typically comprise automated equipment, such as various measuring equipment, telemetry equipment, monitoring systems, tracking and tracing devices, in-vehicle safety systems, vehicle maintenance systems, road sensors, digital billboards, point of sale (POS) terminals, remote control systems and the like. MTC devices can be implemented as a part of a (generally) stationary apparatus such as vending machines, roadside sensors, POS terminals, although some MTC devices can be embedded in non-stationary apparatus (e.g. vehicles) or attached to animals or persons to be monitored/tracked.
For simplicity, the present application refers to MTC devices in the description but it will be appreciated that the technology described can be implemented on any communications devices (mobile and/or generally stationary) that can connect to a communications network for sending/receiving data, regardless whether such communications devices are controlled by human input or software instructions stored in memory. In particular, the term ‘MTC device’ is intended to cover any communications device implementing MTC functionality, such as mobile telephones/user equipment (UE) running an MTC application.
In order to be able to communicate with each other, a Radio Resource Control (RRC) connection needs to be established between an MTC device and the radio access network (RAN), i.e. a base station of that RAN currently serving the MTC device. Under normal circumstances, an RRC connection for a particular MTC device is usually released by the RAN/base station due to inactivity of the MTC device. In some cases, an existing RRC connection may also be released due to a communication failure, authentication error, and/or the like. An RRC connection may also be released upon a request from a core network entity (e.g. the MME) and/or the MTC device itself (e.g. when it is being switched off).
Inactivity of a communications device is determined by the serving base station at the expiry of a so called ‘user inactivity’ timer, when the base station releases the RRC connection for the inactive communications device. Upon expiry of the user inactivity timer, the base station also notifies the core network about the release of the RRC connection using a so-called UE context release procedure, which is described in more detail in section 5.3.5 of the 3GPP TS23.401 standards specification.
Since the use and the length of the user inactivity timer are implementation dependant, details of these are not provided in the above specification. However, the release of a RRC connection for inactivity of an MTC device is typically carried out as follows. The base station runs a user inactivity timer (per RRC connection) during the existence of an RRC connection between the base station and the MTC device. Whenever the MTC device sends or receives data, the base station re-starts the user inactivity timer associated with that MTC device (i.e. the RRC connection used by the MTC device). When the user inactivity timer expires (i.e. it reaches a predefined value or it reaches zero if counting down from the predefined value), the base station initiates the RRC connection release procedure referred to above.
MTC devices connect to the network whenever they have data to send to or receive from a remote ‘machine’ (e.g. a server) or user. MTC devices use communication protocols and standards that are optimised for mobile telephones or similar items of user equipment (UEs). However, MTC devices, once deployed, typically operate without requiring human supervision or interaction, and follow software instructions stored in an internal memory. MTC devices might also remain stationary and/or inactive for a long period of time. The specific network requirements to support MTC devices have been dealt with in the 3GPP TS 22.368 standard, the contents of which are incorporated herein by reference.
In accordance with section 7.2.5 of TS 22.368, MTC devices can benefit from a so-called ‘small data transmissions’ functionality, intended for the sending and/or receiving of small amounts of data (typically in the order of 1K (1024) octets of data). In order to implement this feature, the system supports transmissions of small amounts of data with minimal impact on e.g. signalling overhead, network resources, and delay for reallocation of resources, regardless whether or not the MTC device is attached to the network before transmitting data. The network counts the number of small data transmissions per subscription e.g. for charging or statistical purposes. This allows charging and accounting of small data transmissions between network operators on a bulk basis.
More recently, optimisation of small data transmissions (including keep-alive messages from smartphones) has been considered by 3GPP as part of the MTCe-SDDTE (Small Data transmission and Device Triggering Enhancements) work item, published under section 5.1 of 3GPP TR23.887—the contents of which are incorporated herein by reference.